Natural gas liquefaction method with enhanced propane recovery

ABSTRACT

The liquefaction method provides fractionation of the natural gas with ethane recycle in order to enhance propane recovery and to increase the critical pressure of the gas to be liquefied. 
     The natural gas is partly liquefied by cooling in E 1 , then separated in fractionating column  2  into a methane-rich stream and a stream rich in hydrocarbons heavier than methane. 
     The methane-rich stream is partly liquefied in  3 , then the condensates separated in drum  4  are recycled to the top of column  2  through line  7 . The gas fraction from drum  4  is liquefied in exchanger E 2  to produce the liquefied natural gas. 
     The stream rich in hydrocarbons heavier than methane is separated in the deethanizer into an ethane-enriched fraction and heavier hydrocarbons. 
     According to the invention, the ethane-enriched fraction is at least partly liquefied, then part of the liquefied ethane is recycled to separating drum  4  or reflux line  7.

FIELD OF THE INVENTION

The present invention relates to the sphere of natural gas liquefaction.

BACKGROUND OF THE INVENTION

Raw natural gas mainly comprises methane, as well as various constituents such as water, hydrogen sulfide, carbon, dioxide, mercury, nitrogen and light hydrocarbons comprising generally two to six carbon atoms. Some of these constituents such as water, hydrogen sulfide, carbon dioxide and mercury are pollutants that are removed upstream from the natural gas liquefaction stages. The hydrocarbons heavier than methane are condensed and recovered as natural gas liquids that can be upgraded.

The natural gas liquids are separated from the methane by means of a fractionating column and by cooling and partial liquefaction of the natural gas. The gas obtained at the top of the fractionating column is intended to be liquefied in order to produce the liquid natural gas. Operation at very high pressure allows to limit the energy required for liquefaction. However, the operating pressure of the fractionating column is limited by the critical pressure of the mixture to be separated.

The goal of the present invention is to enhance propane recovery and to increase the critical pressure of the gas to be liquefied in order to achieve fractionation at a higher pressure, thus decreasing the energy required for liquefaction. The invention consists in recycling an ethane stream to the reflux line of the fractionating column or to the fractionation reflux drum.

SUMMARY OF THE INVENTION

In general terms, the invention describes a method of liquefying a natural gas wherein the following stages are carried out:

a) partly liquefying the natural gas by cooling,

b) feeding the partly liquefied natural gas into a fractionating column so as to obtain a methane-enriched gas fraction and a methane-depleted liquid fraction,

c) cooling the gas fraction up to partial liquefaction, then feeding the cooled gas fraction into a separating drum so as to separate a gas phase and a liquid phase,

d) recycling at least part of the liquid phase to the fractionating column as reflux,

e) separating the liquid fraction so as to obtain an ethane-enriched fraction and at least one fraction enriched in compounds heavier than ethane,

f) recycling at least part of the ethane-enriched fraction by carrying out at least one of the following operations:

-   -   feeding said part of the ethane-enriched fraction into said         separating drum,     -   prior to stage d), mixing said part of the ethane-enriched         fraction with said liquid phase,

g) liquefying the gas phase obtained in stage c) by cooling, then by expansion, so as to produce a liquid natural gas.

According to the invention, the ethane-enriched fraction obtained in stage e) can comprise at least 90% by mole of ethane.

In stage f), said at least part of the ethane-enriched fraction can be recycled at a flow rate ranging between 5% and 20% of the flow rate of ethane contained in said natural gas.

The method according to the invention can operate under the following conditions:

the fractionating column can work at a pressure ranging between 40 bars and 60 bars,

in stage a), the natural gas can be cooled to a temperature ranging between 0° C. and −60° C., and

in stage c), the gas fraction can be cooled to a temperature ranging between −45° C. and −70° C.

According to a first option, in stage e), the liquid fraction can be separated in a deethanization column, said ethane-enriched fraction being obtained at the top of the deethanization column, the fraction enriched in compounds heavier than ethane being obtained in the bottom of the deethanization column. Furthermore, the ethane-enriched fraction can be at least partly liquefied, part of the ethane-enriched liquid fraction being introduced at the top of the deethanization column as reflux, another part of the ethane-enriched liquid fraction being recycled according to stage f). According to the first option, the deethanization column can work at a pressure ranging between 20 and 35 bars, and said ethane-enriched fraction can be at least partly liquefied by cooling to a temperature ranging between −5° C. and 10° C.

According to a second option, in stage e), the liquid fraction can be separated in a demethanization column so as to obtain a methane-enriched gas stream and a liquid stream enriched in compounds heavier than methane, then the liquid stream can be separated in a deethanization column, said ethane-enriched fraction being obtained at the top of the deethanization column, the fraction enriched in compounds heavier than ethane being obtained in the bottom of the deethanization column. Furthermore, the ethane-enriched fraction can be at least partly liquefied, part of the ethane-enriched liquid fraction being introduced at the top of the deethanization column as reflux, another part of the ethane-enriched liquid fraction being recycled according to stage f). According to the second option, a portion of the liquid phase obtained in stage c) can be introduced at the top of the demethanization column as reflux. According to the second option, the demethanization column can work at a pressure ranging between 25 and 40 bars, the deethanization column can work at a pressure ranging between 20 and 35 bars, and said ethane-enriched fraction can be at least partly liquefied by cooling to a temperature ranging between −5° C. and 10° C.

BRIEF DESCRIPTION OF THE FIGURES

Other features and advantages of the invention will be clear from reading the description hereafter, with reference to the accompanying figures, wherein:

FIG. 1 diagrammatically shows a liquefaction method with fractionation,

FIGS. 2 to 5 diagrammatically show various embodiments of the invention.

DETAILED DESCRIPTION

In FIG. 1, the natural gas flowing in through fine 1 may have first been purified of impurities such as water, hydrogen sulfide, carbon dioxide and mercury. The natural gas is fed into heat exchanger E1 in order to be cooled until partial liquefaction. In E1, the natural gas can be cooled to a temperature ranging between 0° C. and −60° C. In E1, cooling is carried out by means of closed cooling circuit C1 that works by compression and expansion of a cooling fluid.

The partly liquefied stream from E1 is fed into fractionation zone F. The present invention provides various embodiments for zone F, described in connection with FIGS. 2 to 5. The reference numbers of FIGS. 2 to 5 identical to those of FIG. 1 designate the same elements.

The natural gas liquids are discharged in form of one or more streams LGN. The methane 5 obtained in zone F is subcooled in exchanger E2 until complete liquefaction. In E2, cooling is carried out by means of closed cooling circuit C2 that works by compression and expansion of a cooling fluid. The liquid natural gas under pressure is discharged from E2 to be expanded in expansion device V to atmospheric pressure so as to produce liquid natural gas GNL.

In connection with FIGS. 2 and 3, the natural gas 1 is cooled and partly condensed in exchanger E1, then fed into fractionating column 2. Column 2 generally works at a pressure ranging between 40 and 60 bars abs. The vapour obtained at the top of column 2 is partly condensed by condenser 3. The gas phase is separated from the liquid phase in drum 4. Condenser 3 provides cooling to very low temperature, generally between −45° C. and −70° C., by means of a cooling fluid, for example used in heat exchanger E2. Gas phase 5 is sent to heat exchanger E2 to be liquefied. The liquid phase obtained at the bottom of drum 4 is sent back, by means of pump 6, through line 7 to the top of fractionating column 2 as reflux. The temperature at the bottom of the column is controlled by reboiler 12 so as to vaporize the light fractions present in liquid form in the bottom of column 2 and to limit their entrainment in line 8.

The liquid phase obtained at the bottom of column 2 is discharged through line 8 to deethanization column 14. Column 14 can work between 20 and 35 bars abs. Column 14 allows to separate a stream comprising mainly ethane discharged at the top and a stream comprising hydrocarbons heavier than ethane at the bottom. The ethane stream obtained at the top of column 14 is partly or even totally condensed by cryogenic condenser 15 at a temperature ranging between −5° C. and 10° C. The stream obtained at the outlet of condenser 15 is sent to reflux drum 16. If the ethane stream is only partly condensed, an ethane vapour phase is discharged at the top of drum 16. The liquid ethane obtained at the bottom of 16 is pumped by pump 17 and sent through line 18 to the top of column 14 as reflux. A fraction of the liquid ethane obtained at the bottom of drum 16 can be sent to a storage zone through line 20. The temperature in the bottom of column 14 is maintained by reboiler 21 so as to remove a maximum amount of ethane from the C3+cut discharged at the bottom of 14 through line 22. The C3 cut can be separated, for example in a depropanization column.

In connection with FIGS. 2 and 3, the present invention aims to recycle a portion of the stream rich in liquid ethane obtained at the bottom of drum 16 to fractionating column 2. According to the invention, the ethane-rich stream comprises at least 90% by mole, preferably more than 98% by mole of ethane. More precisely, in connection with FIG. 2, a portion of the liquid stream pumped by pump 17 is fed through line 19 into drum 4. Alternatively, in connection with FIG. 3, a portion of the liquid stream pumped by pump 17 is fed into reflux line 7 through line 19. Thus, the liquid phase obtained at the bottom of drum 4 is combined and mixed with the ethane-rich stream flowing in through line 19.

Ethane recycle according to the invention allows to significantly increase the recovery of propane in the bottom of fractionating column 2. In order to obtain a good propane recovery ratio, an ethane-rich stream having an ethane molar flow rate ranging between 5% and 20% by mole of the molar flow rate of ethane contained in the gas to be treated, flowing in through line 1, is recycled.

Furthermore, the delivery of ethane at the top of column 2 allows to slightly increase the critical pressure of the fluid circulating in column 2 and therefore improves the method of operation of the separation.

Besides, ethane recycle also allows to enrich the natural gas discharged at the top of column 2 in ethane and therefore to upgrade the ethane and to increase the calorific value of the natural gas.

The numerical examples given hereafter allow to illustrate the operating mode of the methods described in connection with FIGS. 2 and 3.

The flow rate of the natural gas flowing in through line 1 is 34,000 kmol/h, with the following composition:

Component Composition (% by mole) N2 0.9 C1 90 C2 8 C3 0.5 iC4 0.1 nC4 0.1 iC5 0.05 nC5 0.05 nC6 0.05 nC7 0.05 nC8 0.05 nC9 0.05 Benzene 0.05 Toluene 0.05

The methods work under the following conditions:

-   -   fractionating column 2:     -   pressure: 45 bars abs at the bottom, 44 bars abs at condenser 3,     -   temperature of the natural gas at the inlet: −30° C.,     -   temperature at condenser 3: −65° C.,     -   deethanizer 14:     -   pressure: 27.5 bars abs at the bottom, 27 bars abs at condenser         15,     -   supply temperature: 42° C.,     -   temperature at condenser 15: 3° C.

The recycle ratio, i.e. the molar flow rate of ethane recycled through line 19 in relation to the molar flow rate of ethane contained in the gas flowing in through line 1, is 5%.

The C3 recovery ratio is defined as the ratio of the flow rate of C3 in line 22 to the flow rate of C3 in line 1.

The operating simulations were carried out for the method described in connection with FIG. 2, for the method described in connection with FIG. 3 and for a method without C2 recycle, i.e. a method identical to those shown in FIGS. 2 and 3, except that it does not comprise the C2 recycle marked by recycle line 19.

Method Method of FIG. 2 Method of FIG. 3 without with C2 recycle with C2 recycle Scheme C2 recycle in reflux drum 4 in reflux line 7 C2 flow rate in the 0 136.4 136.4 recycle (kmol/h) C3 recovery ratio (%) 71.1 79.9 84.2 Critical pressure at 56.7 56.9 56.9 fractionation top 5 (bar)

It can be observed that the invention allows to enhance C3 recovery, to slightly depart from the critical conditions in the fractionating column and to enhance the C3 recovery ratio.

In connection with FIGS. 4 and 5, natural gas 1 is cooled and partly condensed in exchanger E1, to a temperature ranging between −60° C. and 0° C., then it is fed into fractionating column 2. Column 2 can operate at a pressure ranging between 40 bars and 60 bars. The vapour obtained at the top of column 2 is partly condensed by condenser 3. The gas phase is separated from the liquid phase in drum 4. Condenser 3 achieves cooling to very low temperature, for example between −45° C. and −70° C., by means of, a cooling fluid, for example used in heat exchanger E2. Gas phase 5 is sent to heat exchanger E2 to be liquefied. The liquid phase obtained at the bottom of drum 4 is sent back by pump 6 through line 7 to the top of fractionating column 2 as reflux.

The liquid phase obtained at the bottom of column 2 is discharged through line 8 to a second fractionating column 9 to perform a second separation between the methane and the hydrocarbons heavier than methane, at a lower pressure than that of column 2. Column 9 can operate at a pressure ranging between 25 bars and 40 bars. A portion of the liquid phase obtained at the bottom of drum 4 is fed to the top of column 9 as reflux. The temperature at the bottom of column 9 is controlled by reboiler 12 so as to vaporize the light fractions present in liquid form in the bottom of column 9 and to limit their entrainment in line 13. Column 9 allows to obtain, at the top, a methane-enriched stream discharged through line 11 and, at the bottom, a stream enriched in hydrocarbons heavier than methane.

The liquid stream obtained at the bottom of column 9 is fed through line 13 into deethanization column 14. Column 14 works at a lower pressure than column 9, for example at a pressure ranging between 20 bars and 35 bars. Column 14 allows to separate a stream comprising mainly ethane discharged at the top and a stream comprising hydrocarbons heavier than ethane at the bottom. The ethane stream obtained at the top of column 14 is partly or even totally condensed by cryogenic condenser 15 at a temperature ranging between −5° C. and 10° C. The stream obtained at the outlet of condenser 15 is sent to reflux drum 16. If the ethane stream is only partly condensed, an ethane vapour phase is discharged at the top of drum 16. The liquid ethane obtained at the bottom of 16 is pumped by pump 17 and sent through line 18 to the top of column 14 as reflux. A fraction of the liquid ethane obtained at the bottom of drum 16 can be sent to a storage zone through line 20. The temperature in the bottom of column 14 is maintained by reboiler 21 so as to remove a maximum amount of ethane from the C3+cut discharged at the bottom of 14 through line 22. The C3+cut can be separated, for example in a depropanization column.

In connection with FIGS. 4 and 5, the present invention aims to recycle a portion of the stream rich in liquid ethane obtained at the bottom of drum 16 to fractionating column 2. More precisely, in connection with FIG. 4, a portion of the liquid stream pumped by pump 17 is fed through line 19 into drum 4. Alternatively, in connection with FIG. 5, a portion of the liquid stream pumped by pump 17 is fed into reflux line 7 through line 19. Thus, the liquid phase obtained at the bottom of drum 4 is combined and mixed with the ethane-rich stream flowing in through line 19.

The numerical examples given hereafter allow to illustrate the operating mode of the methods described in connection with FIGS. 4 and 5.

The flow rate of the natural gas flowing in through line 1 is 34,000 kmol/h, with the following composition:

Component Composition (% by mole) N2 0.9 C1 90 C2 8 C3 0.5 iC4 0.1 nC4 0.1 iC5 0.05 nC5 0.05 nC6 0.05 nC7 0.05 nC8 0.05 nC9 0.05 Benzene 0.05 Toluene 0.05

The methods work under the following conditions:

-   -   fractionating column 2:     -   pressure: 45 bars abs at the bottom, 44 bars abs at condenser 3,     -   temperature of the natural gas at the inlet: −30° C.,     -   temperature at condenser 15: −65° C.,     -   discharge flow rate 10 for reflux of demethanizer 9: 350 kmol/h,     -   demethanizer 9:     -   pressure: 30 bars abs at the bottom, 29.5 bars abs at the top,     -   supply temperature: −41° C.,     -   deethanizer 14:     -   pressure: 27.5 bars abs at the bottom, 27 bars abs at condenser         15,     -   supply temperature: 42° C.,     -   temperature at condenser 15: 3° C.

The recycle ratio, i.e. the molar flow rate of ethane recycled through line 19 in relation to the molar flow rate of ethane contained in the gas flowing in through line 1, is 10%.

The C3 recovery ratio is defined as the ratio of the flow rate of C3 in line 22 to the flow rate of C3 in line 1.

The operating simulations have been carried out for the method described in connection with FIG. 4, for the method described in connection with FIG. 5 and for a method without C2 recycle, i.e. a method identical to those shown in FIGS. 4 and 5, except that it does not comprise the C2 recycle marked by recycle line 19.

Method according Method to FIG. 4 with C2 Method of FIG. 5 without recycle in reflux with C2 recycle Scheme C2 recycle drum 4 in reflux line 7 C2 flow rate in the 0 273 273 recycle (kmol/h) C3 recovery ratio (%) 62.4 76.6 82.6 Critical pressure at 56.7 56.9 56.9 fractionation top 5 (bar)

It can be observed that the invention allows to enhance C3 recovery and to depart from the critical conditions in the fractionating column, especially when the recycle is sent back through the reflux line. 

1) A natural gas liquefaction method wherein the following stages are carried out: a) partly liquefying the natural gas by cooling, b) feeding the partly liquefied natural gas into a fractionating column so as to obtain a methane-enriched gas fraction and a methane-depleted liquid fraction, c) cooling the gas fraction up to partial liquefaction, then feeding the cooled gas fraction into a separating drum so as to separate a gas phase and a liquid phase, d) recycling at least part of the liquid phase to the fractionating column as reflux, e) separating the liquid fraction so as to obtain an ethane-enriched fraction and at least one fraction enriched in compounds heavier than ethane, f) recycling at least part of the ethane-enriched fraction by carrying out at least one of the following operations: feeding said at least part of the ethane-enriched fraction into said separating drum, prior to stage d), mixing said at least part of the ethane-enriched fraction with said liquid phase, g) liquefying the gas phase obtained in stage c) by cooling, then by expansion, so as to produce a liquid natural gas. 2) A method as claimed in claim 1, wherein said ethane-enriched fraction obtained in stage e) comprises at least 90% by mole of ethane. 3) A method as claimed in claim 1, wherein, in stage f), said at least part of the ethane-enriched fraction is recycled at a flow rate ranging between 5% and 20% of the ethane flow rate of ethane contained in said natural gas. 4) A method as claimed in claim 1, wherein: the fractionating column works at a pressure ranging between 40 bars and 60 bars, in stage a), the natural gas is cooled to a temperature ranging between 0° C. and −60° C., and in stage c), the gas fraction is cooled to a temperature ranging between −45° C. and −70° C. 5) A method as claimed in claim 1 wherein, in stage e), the liquid fraction is separated in a deethanization column, said ethane-enriched fraction being obtained at the top of the deethanization column, the fraction enriched in compounds heavier than ethane being obtained at the bottom of the deethanization column. 6) A method as claimed in claim 5, wherein the ethane-enriched fraction is at least partly liquefied, part of the ethane-enriched liquid fraction being introduced at the top of the deethanization column as reflux, another part of the ethane-enriched liquid fraction being recycled according to stage f). 7) A method as claimed in claim 6, wherein: the deethanization column works at a pressure ranging between 20 and 35 bars, and said ethane-enriched fraction is at least partly liquefied by cooling to a temperature ranging between −5° C. and 10° C. 8) A method as claimed in claim 1 wherein, in stage e), the liquid fraction is separated in a demethanization column so as to obtain a methane-enriched gas stream and a liquid stream enriched in compounds heavier than methane, then the liquid stream is separated in a deethanization column, said ethane-enriched fraction being obtained at the top of the deethanization column, the fraction enriched in compounds heavier than ethane being obtained in the bottom of the deethanization column. 9) A method as claimed in claim 8, wherein the ethane-enriched fraction is at least partly liquefied, part of the ethane-enriched liquid fraction being introduced at the top of the deethanization column as reflux, another part of the ethane-enriched liquid fraction being recycled according to stage f). 10) A method as claimed in claim 8, wherein a portion of the liquid phase obtained in stage c) is fed to the top of the demethanization column as reflux. 11) A method as claimed in claim 8, wherein: the demethanization column works at a pressure ranging between 25 and 40 bars, the deethanization column works at a pressure ranging between 20 and 35 bars, said ethane-enriched fraction is at least partly liquefied by cooling to a temperature ranging between −5° C. and 10° C. 12) A method as claimed in claim 9 wherein a portion of the liquid phase obtained in stage c) is fed to the tip of the demethanization column as reflux. 